As a representative example of a wireless power transmission system, there is known a power transmission system employing a magnetic field coupling method in which power is transmitted from a primary coil of a power transmitting apparatus to a secondary coil of a power receiving apparatus using a magnetic field. However, in the case where power is transmitted by magnetic field coupling, the magnitude of magnetic flux that passes through each of the coils significantly affects the electromotive force, and thus, high precision is required for the relative positional relationship between the primary coil and the secondary coil. Since coils are used, it is difficult to reduce the sizes of the apparatuses.
On the other hand, there is also known a wireless power transmission system employing an electric field coupling method such as that disclosed in Patent Document 1. In this system, power is transmitted from a coupling electrode of a power transmitting apparatus to a coupling electrode of a power receiving apparatus through an electric field. In the method, the degree of precision required for the relative positional relationship between the coupling electrodes is relatively low, and the coupling electrodes can be reduced in size and thickness.
FIG. 1 is a diagram illustrating the basic configuration of a power transmission system according to Patent Document 1. The power transmission system includes a power transmitting apparatus and a power receiving apparatus. The power transmitting apparatus includes a high-frequency high-voltage generating circuit 1, a passive electrode 2, and an active electrode 3. The power receiving apparatus includes a high-frequency high-voltage load circuit 5, a passive electrode 7, and an active electrode 6. The active electrode 3 of the power transmitting apparatus and the active electrode 6 of the power receiving apparatus come close to each other across a high-voltage electric field region 4, so that the two electrodes are brought into an electric field coupling relationship.
As described above, in wireless power transmission employing an electric field coupling method, an active electrode and a passive electrode are arranged in both a power transmitting apparatus and a power receiving apparatus, and the active electrode and the passive electrode in the power transmitting apparatus need to be respectively coupled with the active electrode and the passive electrode in the power receiving apparatus. The amount of power to be transmitted and transmission efficiency depend on the coupling strengths between the electrodes. In order to strengthen the couplings between the electrodes, reducing the distances between the electrodes and increasing the areas of the electrodes can be considered.
Patent Document 1: Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2009-531009
In general, as a method of improving transmission efficiency of a power transmission system, it is effective to incorporate a low-loss resonance circuit in a power transmission system. The resonance circuit includes an electrostatic capacitance and an inductor in a coupling portion in which a power transmitting apparatus and a power receiving apparatus are coupled to each other. In general, the Q value of an inductor is lower than the Q value of a capacitance, and thus, in the case where an electrostatic capacitance and an inductor are combined with each other as a resonance circuit, the Q value of the resonance circuit is limited by the Q value of the inductor. One problem is how to realize a small-sized and low-loss inductor. As a measure to solve the problem, it is effective to use a piezoelectric device (a piezoelectric resonator or a piezoelectric transformer) as an inductor. However, although a piezoelectric device is small-sized and low-loss, there have been problems in that a piezoelectric device has sharp frequency characteristics, and that an output voltage (the ratio V2/V1 of a voltage V2 of a power receiving apparatus to a voltage V1 of a power transmitting apparatus) significantly varies in the case of load variations.
In addition, also in the case where a piezoelectric device is not used, there has been a problem in that the output voltage varies in response to load variations and drive frequency variations.